1. Field of the Invention
The present invention relates to an illumination system that divides a light flux emitted from a light source into a plurality of partial light fluxes and causes the plurality of partial light fluxes to be superimposed on an identical illumination area. The present invention also pertains to a projection display apparatus that produces substantially uniform, bright projected images using such an illumination system.
2. Discussion of the Background
A projection display apparatus uses a light modulator, or `light valve`, to modulate illumination light, illuminating the light modulator, responsive to image information, and projects the modulated light flux on a screen to display an image. A typical example of the light modulator is a liquid-crystal panel. It is naturally desirable that the image displayed by the projection display apparatus is substantially uniform and bright. For that purpose, the illumination light emitted from an illumination device (illumination system) incorporated in the projection display apparatus should have a high utilization efficiency of light. One proposed technique to enhance the utilization efficiency of the illumination light disposes a plurality of micro lenses corresponding to the respective pixels of the liquid-crystal panel on the light-entering side of the liquid-crystal panel.
FIGS. 15(A) and 15(B) show light fluxes entering a liquid-crystal panel in the case where micro lenses are disposed on the light-entering side of the liquid-crystal panel. More concretely FIGS. 15(A) and 15(B) show the cross section of a liquid-crystal panel 1000 and a micro lens array 1100 including a plurality of micro lenses 1110. The liquid-crystal panel 1000 includes liquid-crystal layers 1010 that are surrounded by light shielding layers 1020, which are referred to as the `black matrixes`, in a lattice configuration. The micro lens array 1100 is disposed on the light-entering side of the liquid-crystal panel 1000 in such a manner that the center of one liquid-crystal layer 1010 corresponding to each pixel of the liquid-crystal panel 1000 substantially coincides with the optical axis of one micro lens 1110. As shown in FIG. 15(A), the light flux, which enters the micro lens 1110 substantially in parallel with the optical axis of the micro lens 1110, is condensed by the micro lens 1110 to pass through the liquid-crystal layer 1010. This arrangement ensures the utilization of such light fluxes that would be shielded by the light shielding layers 1020 in the structure without the micro lenses 1110. The micro lenses accordingly work to enhance the utilization efficiency of light.
The micro lens 1110 also condenses the light flux that enters the micro lens 1110 obliquely to the optical axis of the micro lens 1110 as shown in FIG. 15(B). Part of this light flux, however, does not pass through the liquid-crystal layer 1010 but is shielded by the light shielding layer 1020. In this case, the use of the micro lenses worsens the utilization efficiency of light. This phenomenon is more significant in the case of the greater angle of the light flux to the optical axis (that is, the incident angle).
The smaller incident angle of light into the liquid-crystal panel relieves the above problem and improves the utilization efficiency of light. In the structure without micro lenses, the smaller incident angle of light into an optical element other than the liquid-cystal panel (for example, a projection lens for causing the modulated light flux emitted from the liquid-crystal panel to be projected on a screen) would improve the utilization efficiency of light in the optical element and thereby improves the utilization efficiency of light in the whole projection display apparatus.
One possible method of decreasing the incident angle of light into an illumination area is to lengthen the optical path between a light source and the illumination area (especially the optical path between the light source and an optical element immediately before the illumination area). This method, however, undesirably increases the size of the whole illumination system.
In an optical integrator system, a light flux emitted from the light source is divided into a plurality of partial light fluxes, and the plurality of partial light fluxes are superimposed on the illumination area. It is accordingly difficult to decrease the incident angle of light into the illumination area without significantly lengthening the optical path in the illumination system including the optical integrator system.